The present invention relates to mode converters.
An optical mode converter is a coupling device for converting an optical signal from a first propagation mode (e.g., LP01) to a second propagation mode (e.g., LP02). Mode converters typically comprise a single-mode step-index optical fiber for supporting the propagation of an optical signal at the fundamental mode (i.e., LP01) and a multi-mode step-index optical fiber for supporting the propagation of an optical signal at one or more other modes (e.g., LP02). A portion of the cladding is removed from each of the optical fibers. As such, the optical fibers may be spaced at a distance, thereby allowing an evanescent field from an optical signal propagating through one of the optical fibers to couple into the second optical fiber of the mode converter. Once coupled into the second optical fiber, the optical signal propagates at the mode supported by the second optical fiber.
Mode converters have been employed in various applications requiring an interface between two waveguides supporting different propagation modes. One such application for mode converters is in dispersion compensation systems. Dispersion compensation systems remove overlaps created by the broadening of optical signals propagating through a carrier optical waveguide. One known dispersion compensation system includes a specially designed multi-mode optical waveguide supporting a number of propagation modes (e.g., LP02)xe2x80x94each supported mode being higher than the fundamental mode (i.e., LP01). In contrast, a typical carrier optical waveguide is a single-mode fiber supporting the fundamental propagation mode. Consequently, a mode converter is needed to couple the carrier optical waveguide to the specially designed multi-mode optical waveguide in such a way that an optical signal propagating through the carrier optical waveguide at the fundamental mode may be converted to one of the propagation modes supported by the specially designed multi-mode optical waveguide.
Presently, commercially available mode converters are only operable over a narrow rangexe2x80x9410 nm or lessxe2x80x94of wavelengths. This narrow range of present day mode converters has now become a problem. In particular, to accommodate the explosive growth of voice and data traffic, industry has continued to increase the number of available channels in a wavelength division multiplexed (xe2x80x9cWDMxe2x80x9d) system. To support this increase in WDM channels, a growing need has arisen to compensate for dispersion over a wider range of wavelengths, and thus to have mode converters operable over a wider range of wavelengths than has been achieved to this point.
In accordance with the present invention, our mode converter includes a first and a second waveguide, each supporting different modes of propagation, while having matching group index characteristics. We have recognized that by matching the group index characteristics of the first and second optical waveguides, the operable range of wavelengths for a mode converter may be expanded.
In another embodiment of the present invention, we match the slope of an LP01 mode propagation constant of a single-mode optical fiber with the slope of an LP02 mode propagation constant of a multi-mode optical fiber to match the group index characteristics of both fibers.
In yet another embodiment of the present invention, the matched group index characteristics are approximately within (+/xe2x88x92) five (5) percent of each other. We estimate that by matching the group index characteristics of the single mode and multi-mode fibers as such, our mode converter may operate over a range of at least 40 nm, in contrast with the 10 nm operable range of present day mode converters. It will be apparent to skilled artisans upon reviewing the instant disclosure that the more precisely the group index characteristics are matched, the greater the coupling efficiency of the mode converter.